Controlling the braking system in a vehicle is described in German Patent Application No. DE 195 10 746, where the action of the brakes is distributed between at least one front and one rear wheel by adjusting at least the braking pressure in the brakes on one rear wheel in order to establish a difference between the braking pressure on the front wheel and that on the rear wheel. The resulting difference can be diminished only by the driver of the vehicle decreasing the front-wheel braking pressure. The valve or valves employed to establish the braking-pressure difference in a system of this nature can be subjected to excessive thermal stress. It is also possible for the driver of the vehicle to discontinue the operation of the vehicle (by turning off the ignition and accordingly interrupting the supply of current or voltage) while still actuating the brake. In this event the established difference will cause the brake pedal to "give way" when the ignition is turned off, meaning that the resistance perceived by the driver to be exerted by the brake pedal will be noticeably decreased by the sudden pressure adjustment, which is unpleasant for the driver.
European Patent No. EP 0 509 237 (corresponding to U.S. Pat. No. 5,281,012) describes electronic braking force distribution.
A method and device for controlling braking pressure by actuating a solenoid valve is described in German Patent Application No. DE 196 20 037.7 (not a prior publication). This valve comprises a coil and a movable armature. Current or voltage is supplied to the coil to move the armnature. The supply is synchronized to a prescribable pulse duty factor, allowing the valve to be operated as a switching valve, turning off and on in synchronization. This approach ensures smooth braking-pressure variation.
A system for establishing the distribution of braking force between front and rear wheels is described in German Patent Application No. DE 195 11 152. This system is particularly intended for emergency operation, where a controller increases and decreases pressure on at least the rear-wheel brakes so that the alternating increase and decrease generate a saturation pressure on the rear-wheel brakes at a level below that of an admission pressure previously established by the driver. The components are protected from thermal stress when the driver applies the brakes for a long period of time by the emission subsequent to a prescribable maximal interval of a series of pressure-increasing pulses that ramp the rear-wheel braking pressure up to the admission pressure prescribed by the driver. The series of pressure-increasing pulses are generally perceived by the driver of the vehicle in the form of unpleasant pulsations in the brake pedal. An attempt is also made to ensure that the driving safety-enhancing difference between the braking pressure on the front wheel and that on the rear wheel will not decrease until it is certain that the decrease will not lead to driving-critical situations.
An object of the present invention is to ensure pleasant and safe braking-action distribution.